1. Field of the Invention
The present invention relates to the design of microwave component. A microwave component has the characteristic that its size is near the wavelength order with respect to operating frequency, and therefore it is necessary to employ the transmission line theory and electromagnetic field theory instead of AC network theory. This invention is to obtain the right coupling amount between interesting resonators by solving the electromagnetic field problem of the high frequency structure.
2. Description of the Related Art
Wireless communication is an important field in modern communication industry. Telecom related companies compete against one another to obtain channel resources. Because of limited channel resources, every telecom service provider is trying hard to fully utilize the limited bandwidth resource by increasing the communication capacity and improving the communication quality. Because the receiving and transmitting channels and channels of different operation systems are close to one another, they must be well isolated to maintain good communication quality. In order to fully utilize the limited bandwidth resource, the demand for high performance filters or duplexers is heavy. A cross-coupling design is usually used to increase the degree of isolation under a limited range. FIG. 4 is a schematic drawing showing the arrangement and a regular filter without cross coupling, the arrangement of a cross-coupling filter, and a frequency response curve comparison chart obtained from the regular filter without cross coupling and the cross-coupling filter. The part A in FIG. 4 shows resonators coupled to one another without through cross coupling. The part B in FIG. 4 shows resonators coupled together through a cross coupling technique. As illustrated, the so-called cross coupling is to insert a coupling path B3 in between two resonators that are not abutted against each other so that the cross coupling filter B has a frequency response steeper than the frequency response obtained from the regular filter A, achieving the desired high degree of isolation. In FIG. 4, part C shows the frequency response curve comparison chart obtained from the regular filter without cross coupling and the cross-coupling filter. As illustrated, A1 and A2 are frequency responses obtained at different channels from the filter A without cross coupling; B1 and B2 are frequency responses obtained at different channels from the filter after insertion of the cross coupling path. B1 stands for a low frequency channel filter, its steep response occurs at the right side, and its cross coupling excitation is same as the main coupling. This coupling is called in-phase coupling. B2 stands for a high frequency channel filter, its steep response occurs at the left side, and its cross coupling excitation is reversed to the main coupling. This coupling is called reverse-phase cross coupling. Therefore, controlling the amount of cross coupling and its phase effectively achieves the desired high degree of isolation among channels.
It is relatively easier to produce an in-phase cross coupling structure because its structure is similar to the main coupling. Normally, an opening is made on the partition wall between resonators to achieve a coupling, and an adjustment screw is provided between resonators to adjust the amount of coupling. As for reverse-phase cross coupling, it is not so straightforward as in-phase cross coupling. FIG. 5 illustrates a fixed type reverse-phase cross coupling structure and an adjustable reverse-phase cross coupling structure according to the prior art. As shown in part A in FIG. 5, a rod conductor D1 is mounted with an insulative material D2 and set between two resonators D6 to excite reverse-phase cross coupling. The amount of cross coupling is determined subject to the length of the rod conductor D1. However, this design of reverse-phase cross coupling structure D is still not satisfactory in function. When wishing to modify the amount of cross coupling, the cover must be detached from the cavity housing, and then affixed to the reverse-phase cross coupling structure after replacement of the rod conductor D1 with a different length of rod conductor. This procedure may be repeated several times before the accurate length of rod conductor is installed. This adjustment procedure is complicated. Further, frequently dismounting and mounting the cover may damage the threads of the mounting screw holes, resulting in low installation tightness. In part B in FIG. 5, a thin-film circuit board D3 is set between two resonators D6, and an adjustment screw D5 is disposed adjacent to the bar conductor D4 that is formed on the thin-film circuit board D3 through an etching technique. By means of rotating the adjustment screw D5 to perturb the EM field, thereby adjusting the coupling amount. The reverse-phase cross coupling structure D shown in part B in FIG. 5 allows quick adjustment of the coupling amount without detaching the cover, however this design still has drawbacks as follows:
1. This design of reverse-phase cross coupling structure requires installation of an additional circuit board.
2. The installation of the additional circuit board requires much time and labor. Improper installation position of the circuit board affects the performance of the filter, lowering the reliability of the product.
Therefore, it is desirable to provide a reverse-phase cross coupling structure that eliminates the aforesaid drawbacks.